1. Field of the Invention
This invention relates to a high frequency link power converter apparatus for causing natural commutation based on voltage of a phase-advancing capacitor used as a high frequency power source.
2. Description of the Related Art
The applicant of this invention proposed a AC motor driver (Japanese Patent Application No. 61-165028 corresponding to U.S. Pat. No. 4,760,321 issued on July 26, 1988) as one application example of a high frequency link power converter apparatus.
In the AC motor driver, two AC-AC converters naturally commutated by voltages applied to phase-advancing capacitors which serve as high frequency power sources. The first AC-AC converter controls its input current supplied from an AC power source so that the crest value of voltage applied to the phase-advancing capacitor can be kept, at a constant level. The second AC-AC converter performs frequency conversion using the phase-advancing capacitor as the high frequency power source and supplies sinusoidal current of variable voltage and variable frequency to the AC motor.
In the above device, the input current supplied from the AC power source can be controlled to have a sinusoidal waveform which is inphase with the power source voltage, and the operation can be attained with input power factor=1 and with less higher harmonics of input current. Current supplied to the AC motor can be controlled to have a sinusoidal waveform, making it possible to perform the operation without any torque ripple. In this case, the maximum output frequency can be set at as high as several hundreds Hz. Thus, a veriable-speed AC motor of very high speed and large capacity can be provided.
However, the high frequency link power converter apparatus of the above Japanese Patent Application has the following problems.
In principle, in the apparatus of the above Japanese Patent Application, the circulating current of the first AC-AC converter naturally increases or decreases so as to coincide the frequency and phase of voltage applied to the phase-advancing capacitor with those of a reference signal (high frequency 3-phase power source voltage) supplied to the phase controller of the cycloconverter. However, in practice, a phase difference due to circuit loss or the like occurs between the voltage applied to the phase-advancing capacitor and the reference voltage. As a result, voltage actually applied to the input terminal of each converter becomes inconsistent with phase reference voltage for determining the firing phase of the converter, making it impossible to generate necessary output voltage. Then, the phase input signal is deviated correspondingly, narrowing the controllable range and causing the power source control to be made nonlinear and saturated.
When the load of the high frequency link power converter apparatus is abruptly changed, the phase of voltage applied to the phase-advancing capacitor varies with respect to the reference voltage, but the rate of attenuation of this variation is slow in the apparatus of the above Japanese Pat. Application. At this time, commutation failure of natural commutation will be caused by such variation in the phase of voltage applied to the phase-advancing capacitor, and overcurrent, etc., incurred by the commutation failure, will, negatively affect the element.